When leaders from Exxon Mobil and BP gathered last month with other fossil-fuel executives to declare they were serious about climate change, they cited progress in curbing an energy-wasting practice called flaring — the intentional burning of natural gas as companies drill faster than pipelines can move the energy away.

But in recent years, some of these same companies have significantly increased their flaring, as well as the venting of natural gas and other potent greenhouse gases directly into the atmosphere, according to data from the three largest shale-oil fields in the United States.

The practice has consequence for climate change because natural gas is a potent contributor to global warming. It also wastes vast amounts of energy: Last year in Texas, venting and flaring in the Permian Basin oil field alone consumed more natural gas than states like Arizona and South Carolina use in a year.

Exxon’s venting and flaring has surged since 2017 to record highs, both in absolute terms and as a proportion of gas produced, the numbers show. Exxon flared or vented 70 percent more gas in 2018 than it did the previous year, according to the data, bringing an end to several years of improvements.

Flaring and venting are legal under state laws, and oil companies acknowledge the practices are wasteful. Typically, venting or flaring occur because there aren’t pipelines close enough to a well to capture and transport the gas, or because gas prices are so low that it’s cheaper to discard the gas than to try to sell it. Venting can also occur during equipment breakdowns.

Since 2011, the period for which reliable numbers are available, Exxon has flared or vented more gas overall than any other operator in the three oil fields, which include the Eagle Ford and Permian basins in the Southwest, and the Bakken straddling the Canadian border. Companies often treat natural gas as a byproduct when drilling for oil, which is far more lucrative.

The data also shows that BP this year acquired some of the most polluting sites in the Permian and then allowed flaring and venting to increase. BP burned off 17 percent of the gas it produced in the Permian between April and June of this year (the first full quarter after the acquisition) making it the worst performer in percentage terms among the top 50 producers. In the year-earlier quarter, BP had burned only 10 percent.

When asked about its practices, Exxon Mobil said it was committed to a 25 percent reduction in flaring globally by 2020, compared to 2016 levels, to address environmental concerns.

BP said it was investing in upgrades at its Permian wells that would eliminate much of its flaring. The company also said it was not putting new wells in the area unless they had access to a gas pipeline, reducing the need to burn off or vent excess natural gas.

The analysis provides one of the clearest pictures to date of the companies behind the vast emissions of natural gas that have resulted from America’s shale oil boom, fueled by the use of hydraulic fracturing, or fracking, to unlock fossil fuels from shale rock.

Last year, operators across the three basins together flared or vented a record 320 million cubic feet of gas, more than 40 percent above levels seen just five years ago. The pace for the first two quarters of 2019 has been even higher.

Rystad Energy, an energy analytics company that compiles industry data from state-level corporate disclosures, provided the venting and flaring data to The New York Times, which performed an independent analysis. Separately, an organization affiliated with Greenpeace, Unearthed, also did its own initial analysis of a similar set of data.

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A Chevron site near Midland, Tex.Credit...Jessica Lutz/Reuters

But flaring releases carbon dioxide, a major greenhouse gas, into the atmosphere, where it traps the sun’s heat, driving climate change. Venting directly emits methane, an even more potent greenhouse gas in the shorter term.

Both practices are “a tremendous waste of a natural resource,” said Riccardo Puliti, global director for energy at the World Bank, which leads a global public-private partnership that aims to reduce the practice. The World Bank estimates that flaring last year emitted more than 350 million tons of carbon dioxide globally, equivalent to the greenhouse gas emissions of almost 75 million cars.

“We can’t afford for this to continue unabated,” Mr. Puliti said.

Shale oil has made the United States the world’s largest oil producer. But shale wells tend to dry up more quickly than conventional oil fields. That means producers must drill constantly to keep their oil production steady, while venting or flaring off the gas before pipelines can catch up.

The increase comes even as a group of the world’s biggest oil companies, including Exxon and BP, announced last month on the sidelines of the United Nations Climate Action Summit in New York that they had made progress in reducing global methane emissions, and that its members were on track to meet a target of keeping methane emissions to below 0.25 percent of global production by 2025. BP separately said this year that it had already met an even more stringent target of 0.2 percent.

But environmental groups have urged companies to provide a better accounting of how they measure their emissions and tally those percentages, which are not easily calculated from flaring or venting statistics.

“They’re saying, ‘Here’s our number. Trust us,’” said Ben N. Ratner, a senior director with the Environmental Defense Fund, a group that works with oil companies to track and reduce methane. “There’s been no breakdown of how they arrived at that number. And we don’t have all the facts, the transparency, to assess whether that’s accurate or not.”

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Natural gas, a potent contributor to global warming, is often seen as unprofitable. Video by Jessica Lutz for The New York TimesCredit

When an energy company strikes oil and begins to pump, less-valuable natural gas comes up alongside the oil. That gas could be gathered into pipelines and sold, but drilling has far outpaced pipeline construction, particularly in the booming oil fields of the Permian and Bakken.

Rather than delay drilling, producers will choose to vent or flare.

Many smaller oil producers flare or vent 100 percent of the gas their wells produce, the data shows. In those cases, “gas becomes more like a liability,” said Artem Abramov, an industry analyst at Rystad Energy. “It’s just much cheaper for companies to get rid of it.”

The shale-oil producer Exco Resources highlights this trend. This year it applied with Texas regulators to flare almost all the gas it produced in South Texas, even though a pipeline already exists to move it away, because it is cheaper to release the gas than pay the fees to pipe it off and sell it.

In an unusual showdown, the pipeline’s operator, Williams Companies, is now challenging Exco, saying that allowing it to flare even though its wells were already hooked up to a pipeline would lead to “unnecessary and wasteful flaring of billions of cubic feet of natural gas.” Still, Texas regulators granted Exco’s flaring permit. Williams is urging the state to reconsider.

Exco declined to comment.

The Trump administration, as part of its wide-ranging rollback of regulations to fight climate change, is moving to eliminate Obama-era rules that would have required oil and gas producers to more aggressively detect and fix gas leaks, and to rein in flaring or venting. Fossil fuel companies have been divided on how to respond, with some arguing the rules were too costly.

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Pump jacks in the Permian Basin, near Goldsmith, Tex.Credit...Tamir Kalifa for The New York Times

Among other worst performers are independent petroleum producers, like Marathon Oil, that drill almost exclusively for oil and treat the natural gas that comes up alongside it as a byproduct.

Last year, Marathon Oil vented or flared almost half of the gas produced at its wells in the Bakken. In this year’s first half, that proportion increased to more than half. And since 2018, Marathon Oil’s overall venting and flaring has surpassed even Exxon’s.

A Marathon spokeswoman said the company was “actively pursuing" ways to reduce its emissions.

Chevron, on the other hand, has demonstrated more discipline over the past three years, keeping flaring and venting to less than 3 percent of the gas it drilled, the data shows. Analysts said the company appeared to have stricter internal rules that discourage drilling in areas that offer few prospects of economically recovering the natural gas produced.

“We built a strategy early in our Permian development that, whenever possible, we would not flare to produce,” Veronica Flores-Paniagua, a Chevron spokeswoman, said in a statement.

In an interview, Brian Pugh, chief innovation officer at BP’s onshore oil and gas business in the United States, said the company was investing in new, centralized gathering and compression facilities in the Permian that would enable it to capture and sell more gas instead of flaring and venting it. Nearly all new wells there will connect to the new facilities, the first of which will go online in the first half of 2020, Mr. Pugh said.

“If you look at us this time next year, we’ll be starting to look very, very different,” he said.